Semiconductor elements using group-III nitride semiconductors have light-emitting and light-receiving capabilities for light in the range from visible to ultraviolet. Part of the semiconductor elements have been in practical use.
Since the optical transitions of group-III nitride semiconductors are direct transitions, high-efficiency radiative recombination can occur. The transition energies thereof widely range from 2 to 6.2 eV. Group-III nitride semiconductors are being developed as high-efficiency light-emitting element material for semiconductor lasers (LDs) and high-intensity visible light-emitting elements (LEDs). Gallium nitride (GaN) semiconductors can emit light at a wavelength in the ultraviolet region, as group III-V compound semiconductors. GaN semiconductors are also considered to be capable of replacing existing ultraviolet light sources.
Group-III nitride semiconductors are represented by the general formula In1-x-yAlxGayN (0≦x≦1, 0≦y≦1, 0≦x+y≦1). This includes InN, AlN, GaN, In1-yGayN, AlxGa1-xN, and the like. In the case where only constituent elements are shown while composition ratios (x, y, and the like) are being omitted, the series constituted by the constituent elements is represented. For example, InGaN represents the series generally described as In1-yGayN.
In1-yGayN is one of group-III nitride semiconductors. The band gap energy of In1-yGayN can be changed from 3.4 eV for GaN to 2 eV for InN by changing the In composition 1-y. Accordingly, InGaN can be used for an active layer of a visible LED.
Currently, an LED using an InGaN mixed crystal as a light-emitting layer has been realized. Also in an LD, laser oscillation has been realized in current injection. However, the efficiency of an LED using a crystal with a small In composition 1-y (approximately equal to or less than 0.05) as an active layer is hard to increase because of the difference in light emitting mechanism.
On the other hand, the band gap energy of an AlxGa1-xN semiconductor can be changed between 3.4 eV and 6.2 eV by changing the Al composition x. Although AlxGa1-xN semiconductors have potential as ultraviolet light-emitting materials, it is hard to obtain high-efficiency light emission. This is because AlxGa1-xN semiconductors do not have a specific emission mechanism related to In unlike InGaN semiconductors.
Group-III nitride semiconductors, which are materials emitting light in the range from visible to ultraviolet, are greatly affected by many crystal defects existing in crystals. Injected carriers cause non-radiative recombination in the crystal defects. This results in the decrease in emission efficiency.
For the crystal growth of group-III nitride semiconductors, metal-organic chemical vapor deposition method (also referred to as MOCVD or MOVPE) and molecular beam epitaxy method (MBE) are generally used.
Hereinafter, a known typical method of growing a group-III nitride semiconductor using MOCVD will be described.
An ideal substrate is a favorable one for epitaxial growth and has a small lattice constant difference and a small thermal expansion coefficient difference from the group-III nitride semiconductor. However, such a substrate material is hard to obtain. Accordingly, single-crystal sapphire is mostly used for convenience in terms of stability in a crystal growth atmosphere, a price, and the like.
This sapphire substrate is mounted in a reactor. The temperature of the sapphire substrate is kept at a low temperature between 400° C. and 600° C. In this state, trimethylgallium (TMG), which is organic metal, and ammonia. (NH3) are supplied on the sapphire substrate using hydrogen as carrier gas, thus growing a GaN buffer layer. Thereafter, a crystal layer, e.g., a single-crystal GaN layer, necessary for the structure of an element, such as an LED, is grown. That is, the temperature of the sapphire substrate is raised to 1000 to 1100° C., and ammonia and TMG are supplied on the GaN buffer layer, thus growing the single-crystal GaN layer. In the case where a single crystal having Al as a constituent element, e.g., a single crystal of AlGaN, is grown, trimethylaluminum (TMA) is further added to the raw material to grow this single crystal.
However, this method of growing a group-III nitride semiconductor has a problem that many crystal defects exist in the GaN crystal grown on the sapphire substrate.
One cause thereof is strain due to the lattice constant difference. Another cause thereof is strain due to the thermal expansion difference between the sapphire substrate and the grown layer in the cooling process from the growth temperature to room temperature. These crystal defects need to be reduced in order to manufacture a light-emitting element having high emission efficiency. Particularly in an element emitting ultraviolet light, reducing crystal defects is an important subject.
A growth method in which a patterning mask and lateral growth (in a direction perpendicular to the stacking direction) are combined has been mostly used. This is for reducing crystal defects, i.e., for preventing the propagation of dislocation from a substrate side to a semiconductor layer grown on the substrate.
This method requires a mask formation process and a total of two MOCVD crystal growth processes before and after the foregoing process. Here, one crystal growth process means a series of operations stating with attaching a substrate to an MOCVD system and ending with taking out the substrate from the MOCVD system to the outside after growing a crystal. As a result, there has been a problem that the elongation of the growth process time cannot be avoided and results in the increase in the cost of a light-emitting element.
As an improvement measure for this, there is a technology for reducing crystal defects in one growth process. As shown in FIG. 1, buffer layer 52 is grown on sapphire substrate 51 at a low temperature between 400° C. and 500° C. An undoped GaN layer 53 is epitaxially grown on the buffer layer at a temperature between 1000° C. and 1100° C. However, in this growth method, crystal defects have not been sufficiently reduced.
In order to solve such a problem, there is a technology which reduces crystal defects in one growth process. This is a method in which the propagation of dislocation to an upper layer is prevented by a low-temperature deposited layer (for example, refer to “Motoaki Iwaya, et al., Jpn. J. Appl. Phys. Vol. 37 (1998) pp. L316-L318,” hereinafter referred to as “Literature 1”).
The semiconductor stack disclosed in this literature 1 is shown in FIG. 2A. Buffer layer 62 (a first low-temperature deposited layer) made of AlN, is grown on sapphire substrate 61 at low temperature (400° C.) On buffer layer 62, undoped GaN layer 63 is grown at high temperature (1050° C.). On undoped GaN layer 63, second low-temperature deposited layer 64 made of AlN is grown at low temperature (400° C.). On second low-temperature deposited layer 64, undoped GaN layer 65 is grown at high temperature (1050° C.). This stack can be regarded as a structure in which second low-temperature deposited layer 64 is interposed between two undoped GaN layers 63 and 65 grown at high temperature.
This literature 1 has reported that first and second low-temperature deposited layers 62 and 64 have the effect of reducing crystal defects of GaN layer 63 regardless of whether first and second low-temperature deposited layers 62 and 64 are made of AlN or GaN. Newly providing second low-temperature deposited layer 64 is considered important to the reduction of crystal defects, rather than whether the composition is Al or Ga.